Low tension spark plug



Dec. 3, 1963 A. CANDELISE LOW TENSION SPARK PLUG Filed Jan. 23. 1961 INVENTOR. By fi/fw Q io/ ATTORNEY United States Patent f 3,113,232 LOW TENSION SPARK PLUG Alfred Candelise, Flint, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Jan. 23, 1961, Ser. No. 84,014 5 Claims. (Cl. 313-115) This invention pertains to spark plugs, and particularly to low tension spark plugs of the so-called creepage gap yp It is well recognized that creepage gap spark plugs have a relatively short life due to rapid erosion of the electrodes and/ or semiconductor. It is thought that erosion of the electrodes and the semiconductor is due at least in part to the elevated temperature of the firing end of creepage gap type spark plugs. The present invention relates to an improved spark plug construction of the creepage gap type including means for rapidly conducting the heat away from the firing end of the plug so as to substantially reduce the operating temperature thereof.

Accordingly, among my objects are the provision of a creepage gap type spark plug including means for rapidly conducting the heat away from the firing end thereof; the further provision of a creepage gap type spark plug including a center electrode comprised at least in part of cast silver; and the still further provision of a creepage gap type spark plug comprising coaxially arranged electrodes spaced apart by an annulus of semiconductive material and including a mass of metal having a high coeflicient of thermal conductivity in intimate contact with the semiconductor and the center electrode.

The aforementioned and other objects are accomplished in the present invention by casting silver in the centerbore of both the ceramic insulator and a portion of the semiconductor body. Specifically, in the illustrative embodiment disclosed herein the spark plug comprises a metal shell constituting the ground electrode, a throughbored ceramic insulator, and a semiconductor body in the form of an annulus. A center electrode spindle is suitably secured in the semiconductor body, the lower end thereof projecting outside the semiconductor body and constituting a firing tip. The upper portion of the semiconductor body and the lower portion of the centerbore have silver centrifugally cast therein, the silver being in intimate contact with both the center electrode spindle and the semiconductor body. Accordingly, the silver is able to rapidly conduct heat away from the firing tip of the plug thereby cooling both the center electrode and the semiconductor gap so as to materially reduce erosion and increase the useful life of the plug.

Further objects and advantages will be apparent from the following description, reference being had to the accompanying drawing comprising a side view of the improved spark plug shown partly in section and partly in elevation, and depicting a preferred embodiment of the present invention.

As shown in the drawing, the creepage gap type spark plug includes an outer metal shell having a threaded end at 12 constituting an annular ground electrode. A centerbored ceramic insulator 14 is suitably held in snug relation with the shell 10, the centerbore of the insulator including a center electrode assembly, the upper end of which comprises a terminal screw which receives a terminal nut 16. A stepped annulus 18 of any suitable semiconductive material is arranged within the shell 10 below the ceramic insulator 14. The lower end of the center electrode assembly is constituted by a center electrode spindle 20 which projects beyond the ground electrode 12 and the semiconductor body 18. The composition of the semiconductor may be as disclosed in my Patent 3,113,232 Patented Dec. 3, 1963 2,894,315, it being understood that the composition thereof constitutes no part of the present invention.

In order to maintain the temperature of the firing end of the creepage gap spark plug sufliciently cool to materially reduce erosion of both the electrode and the semiconductor body, a mass of silver 22 is centrifugally cast in the centerbore of the lower portion of the ceramic insulator 14 and the upper portion of the semiconductor body 18, the silver being in intimate contact with both the center electrode spindle 20 and the semiconductor body. The silver is formed by inserting a silver plug into the bore of the insulator after the spindle or firing tip 20 has been positioned. Thereafter the assembly is heated to the melting temperature of the silver and centrifuged so as to cast the silver in situ. To facilitate the intimate contact of the cast silver mass 22 with the center electrode spindle 20, the center electrode spindle is formed with a reduced diameter shank portion 24 which is radially spaced from the inner wall of the annular semiconductor body.

While the precise nature of the creepage gap or low tension type igniter is not fully understood, it is believed that electrical discharge which forms along the surface of the semiconductor between the spaced electrodes ionizes the gas to permit sparking between the center electrode and the ground electrode at a relatively low voltage. This phenomenon is exothermic and thus raises the temperature of the firing end of the plug to a degree at which rapid erosion of both the center electrode spindle and the semiconductor can occur. In addition, of course, the firing tip of the plug is heated by the gases in the combustion chamber into which it extends. By embodying a mass of metal having a high coefficient of thermal conductivity in intimate contact with both the semiconductor body and the center electrode spindle, it. is possible to materially reduce the operating temperature of the firing tip of the plug since this mass of metal rapidly conducts the heat away from the firing end of the plug. In this manner improved low tension spark plugs having a much longer life than those of conventional construction can be made.

It will be understood that while the invention has been described specifically with reference to a preferred embodiment thereof, changes and modifications may be made all within the full and intended scope of the claims which follow:

I claim:

1. A low voltage spark plug including, a tubular metal shell having a lower end constituting an annular ground electrode, an annulus of semiconductive material arranged coaxially within said shell, and a center electrode assembly carried in part by said annulus of said semiconductive material and having a lower end which is radially spaced from said ground electrode so as to define an annular spark gap therebetween, said center electrode assembly being composed at least in part of a metal having a high coeflicient of thermal conductivity which metal is in intimate contact with the annulus of semiconductive material, said material extending from said spark gap to a substantial distance within the shell to provide support for the center electrode assembly and to increase the surface area of contact therewith so as to rapidly conduct heat away from the firing tip of said plug during operation and thereby reduce erosion of said annulus of semiconductive material.

2. A low voltage spark plug including, a tubular metal shell having one end constituting an annular ground electrode, an annulus of semiconductive material arranged coaxially of said shell adjacent said ground electrode, and a centerwire assembly carried at least in part by said annulus of semiconductive material including a center electrode spindle radially spaced from said annular ground electrode to define a spark gap therebetween and a mass of metal having a high coefficient of thermal conductivity in intimate con-tact with said annulus of semiconductive material, said material extending from said spark-gap to a substantial distance within the shell to provide support for the centerwire assembly and to increase the surface area in contact therewith for rapidly conducting heat away from the firing tip of said plug to reduce erosion of said .annulusof semiconductive material.

'3.The spark plug set forth in claim 2 wherein said centerelectrode spindle includes a shank portion of re- 4. The spark plug set forth in claim 2 wherein said mass of metal having a high coeflicient of thermal conductivity comprises cast silver.

5. The low voltage plug set forth in claim 2 including a center-bored ceramic insulator having a lower portion disposed within said tubular shell in axial alignment with said annulus of said semiconductive material, and wherein said mass having a high coefiicient of thermal conductivity extends into the lower portion of the centerbore of said 10 ceramic insulator.

References Cited in the file of thispatent UNITED 'STATES PATENTS 15 2,837,679 Schwartzwalder et al. June 3,1958 2,894,315 Candelise July 14, 1959 2,988,663 Rademacher 'June 13,1961 

1. A LOW VOLTAGE SPARK PLUG INCLUDING, A TUBULAR METAL SHELL HAVING A LOWER END CONSTITUTING AN ANNULAR GROUND ELECTRODE, AN ANNULUS OF SEMICONDUCTIVE MATERIAL ARRANGED COAXIALLY WITHIN SAID SHELL, AND A CENTER ELECTRODE ASSEMBLY CARRIED IN PART BY SAID ANNULUS OF SAID SEMICONDUCTIVE MATERIAL AND HAVING A LOWER END WHICH IS RADIALLY SPACED FROM SAID GROUND ELECTRODE SO AS TO DEFINE AN ANNULAR SPARK GAP THEREBETWEEN, SAID CENTER ELECTRODE ASSEMBLY BEING COMPOSED AT LEAST IN PART OF A METAL HAVING A HIGH COEFFICIENT OF THERMAL CONDUCTIVITY WHICH METAL IS IN INTIMATE CONTACT WITH THE ANNULUS OF SEMICONDUCTIVE MATERIAL, SAID MATERIAL EXTENDING FROM SAID SPARK GAP TO A SUBSTANTIAL DISTANCE WITHIN THE SHELL TO PROVIDE SUPPORT FOR THE CENTER ELECTRODE ASSEMBLY AND TO INCREASE THE SURFACE AREA OF CONTACT THEREWITH SO AS TO RAPIDLY CONDUCT HEAT AWAY FROM THE FIRING TIP OF SAID PLUG DURING OPERATION AND THEREBY REDUCE EROSION OF SAID ANNULUS OF SEMICONDUCTIVE MATERIAL. 